method of selecting a modulation and coding scheme based on adjusted channel quality values

ABSTRACT

In a method for selecting modulation and coding scheme for data transmitted from a transmitter to a receiver, the transmitter determines an estimated channel quality value. The transmitter then adjusts the estimated channel quality value in relation to a distribution of estimated channel quality values, and selects modulation and coding scheme based on the adjusted channel quality value. The invention also extends to a transmitter and a computer program product configured to select modulation and coding scheme accordingly.

TECHNICAL FIELD

The present invention relates to a method and a device for selectingModulation and Coding Scheme (MCS) when transmitting data from atransmitter to a receiver.

BACKGROUND

Link adaptation is a fundamental technique in modern mobilecommunication systems. With link adaptation, the channel coding rate andmodulation scheme is typically chosen based on so-called Channel QualityIndex (CQI) reports. These CQI reports are derived at the receiver toreflect channel quality and interference levels, and subsequentlytransmitted over a signaling channel.

As an alternative, the CQI reports can be estimated directly at thetransmitter. based on the quality of the transmissions received from theintended receiver.

The CQI reports are then used as a basis to select the channel codingrate and modulation scheme to transmit as much user data as possibleusing as little resources as possible. This is achieved by predictingthe resulting block-error probability (BLEP) for each possiblemodulation and coding scheme (MCS). That is for each combination ofmodulation and channel coding the resulting BLEP is calculated and thecurrently best MCS is then selected based on the outcome of thosecalculations. If the channel quality reports provide an accurate measureof the channel quality at the transmission instant, it becomes possibleto maintain tight control over the BLEP when transmitting a data stream.

For several reasons however, the channel quality during transmission isdifferent from the channel quality indicated by the CQI reports. Atleast three factors will cause the actual channel quality to deviatefrom the channel quality in the CQI reports:

-   -   The CQI reports are old, i.e. the channel and/or interference        levels may have changed since the measurement.    -   Estimation errors.    -   The CQI reports are quantized.

These inaccuracies are well-known and attempts have been made to reducethem. Although successful to some extent, the improved estimationmethods leave significant inaccuracies in the CQI estimates resulting inthat the non-optimal modulation and coding scheme is selected fortransmission.

Hence, there exist a need for an improved method and system fordetermining the modulation and coding scheme based on a channel qualityvalue such as a CQI report.

SUMMARY

It is an object of the present invention to overcome or at least reducesome of the problems associated with prior art systems for determining amodulation and coding scheme based on a channel quality value.

This object and others are obtained by the method, transmitter andcomputer program product as set out in the appended claims. Thus, byrealizing that the CQI report or any other channel quality value is notaccurate and instead of treating the values relating to a reportedchannel quality as if they were true, the channel quality values aretreated as stochastic variables with a certain distribution a betterselection of coding and modulation scheme can be achieved. Hence, thelink adaptation is based on link measurement results that take thedistribution of the channel quality values into account.

In accordance with the invention a method is provided for selectingmodulation and coding scheme for data transmitted from a transmitter toa receiver wherein the transmitter determines an estimated channelquality value. The transmitter then adjusts the estimated channelquality value in relation to a distribution of estimated channel qualityvalues, and selects modulation and coding scheme based on the adjustedchannel quality value.

The invention also extends to a transmitter and a computer programproduct configured to select modulation and coding scheme accordingly.

In accordance with one embodiment, the distribution of the channelquality values, such as CQI reports, can be assumed to be known apriori, which is true for the quantization error.

In accordance with one embodiment the distribution of the channelquality values, such as CQI reports, is estimated based on data relatedto a particular transmitter transmitting on a particular link.

Using the modulation and coding scheme selection method in accordancewith the invention will provide an improved selection of modulation andcoding scheme whereby the resulting Block Error Rate (BLER) will bettercorrespond to the block error probability which the transmitter aims atreaching. This in turn provides a more efficient transmitter that needsto utilize less resources.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described in more detail by way ofnon-limiting examples and with reference to the accompanying drawings,in which:

FIG. 1 is a view of a transmission system for transmitting data from atransmitter to a receiver.

FIG. 2 is a view of the resulting BLEP for a first transmission scenariofor different MCSs.

FIG. 3 is a view of the resulting BLEP for a second transmissionscenario for different

MCSs,

FIG. 4 is a view of the resulting BLEP for a third transmission scenariofor different MCSs,

FIG. 5 is a view of the resulting BLEP for a fourth transmissionscenario, and

FIG. 6 is a flowchart illustrating steps performed when selectingmodulation and coding scheme.

DETAILED DESCRIPTION

In FIG. 1 a view of a transmission system 100 is shown. The system 100comprises a transmitter 101 and a receiver 103. The system 100 isconfigured to employ a channel quality value, such as a Channel QualityIndex (CQI) report, for selecting Modulation and Coding Scheme (MCS)when determining how to transmit data from the transmitter 101 to thereceiver 103. The system 100 can typically be a cellular radio systemsuch as a Wideband Code Division Multiple Access (WCDMA) system or anyother cellular radio system. The transmitter 101 can be a radio basestation and the receiver can be a User Equipment (UE) communicating overa radio interface with the radio base station. The receiver is furtherprovided with a unit 105 for generation of a channel quality value suchas a Channel Quality Index (CQI) report based on the quality of datareceived from the transmitter 101 over the channel between thetransmitter and the receiver. The unit 105 is further configured to sendthe CQI reports back to the transmitter 101. The transmitter 101 isprovided with a unit 107 for processing received CQI reports and forselecting a suitable Modulation and Coding Scheme (MCS) based on thereceived CQI reports. The processing of received CQI reports from thereceiver 103 can be performed in accordance with different methods. Themethod used can typically be programmed and stored on a computer programproduct 109 loadable into the unit 107.

In conventional cellular radio systems the CQI estimates are used asaccurate. The underlying assumption is that if the CQI estimates arecorrect on average, the link adaptation will work well on average, i.e.,the Block error probability (BLEP) will be correctly predicted onaverage. In other words, it is acknowledged that the BLEP for a certaintransmission is not as it should have been predict from the CQI report,since an individual CQI report is inaccurate. However, because manyModulation and Coding Scheme (MCS) selections are made, the averageblock error probability for all these transmissions will equal theaverage block error probability predicted from the CQI reports.

However, this has been found to not be true. Hence, although the CQIreports are unbiased estimates of the channel quality, the resultingaverage block error probability will differ from the block errorprobability predicted from the CQI report. Expressed in mathematicalterms:

E└BLEP(C{circumflex over (Q)}I)]┘≠BLEP(E└C{circumflex over (Q)}I┘)

where E[x] denotes the expected value of x.

To exemplify, consider the link adaptation in High Speed Downlink PacketAccess (HSDPA) when 10 HS-PDSCH codes are available. Assume that it isdetermined to evaluate the resulting average BLEP when the linkadaptation targets a 10% BLEP for the transport block sizes supported bythe standard. Assume further that only the CQI inaccuracy caused byquantization effects is taken into account.

Consider a quantization of 0.1 dB. This means that the quantizationerror is uniformly distributed between −0.05 dB and 0.05 dB. (Note thatthe quantization in HSDPA is 1 dB.) In the example given the CQI reportsare thus relatively accurate. The result is shown in FIG. 2. As isclearly seen in FIG. 2, the link adaptation works as intended withaccurate CQI reports with the resulting average BLEP is close to thetarget BLEP, i.e. close to 10%.

Now, the same scenario is investigated, but the quantization isincreased to the level that is actually in the standard: i.e. 1 dB,resulting in a quantization error that is uniformly distributed between−0.5 dB and 0.5 dB. The result is shown in FIG. 3.

From FIG. 3 it is clear that the difference between the target BLEP andthe resulting BLEP is significant. In fact for the majority of the MCSs,the resulting BLEP is around 30% compared to the target BLEP of 10%.Only for the largest transport blocks, it approaches the 10% target.

The situation illustrated above in conjunction with FIG. 3 becomes evenworse when the target is reduced to 1%, as depicted in FIG. 4. If asdepicted in FIG. 4 the aim is a BLEP of 1%, the use the CQI reports asif they were true will result in average BLEPs around 15% for themajority of the MCSs.

One of the reasons for these large deviations between the target BLERand the actual outcome is the effectiveness of the turbo codes. That is,the link performance curves are so steep that the small error caused bythe quantization leads to a dramatic change in the BLEP. Furthermore,the link performance is not symmetric with respect to theSignal-to-Interference Ratio (SIR) errors because in a linear scale, theBLEP increases more for a negative SIR error than it decreases for apositive SIR error.

As an example contemplate the link performance curves of two MCSs,corresponding to transport block sizes of 12048 and 15967 bits,respectively. Referring to FIG. 4 the first MCS has an average BLEP of15%, whereas the second has an average BLEP of 7%. To ease comparison ofthe link performance of the two MCSs. the curve corresponding to thelargest transport block has been translated 3.5 dB. The result is shownin FIG. 5.

From FIG. 5 it can be seen that the two MCSs require 16.65 and 16.95 dB,respectively to achieve 1% BLEP. However, the BLEP increases rapidly asthe SIR is decreased: with only 0.25 dB less SIR, the BLEP increases to20% and 10%. respectively. From these observations. it is fair toconclude that the average BLEP for the second MCS is less sensitive toerrors in the SIR, because the curve is slightly less steep.

From the above examples described in conjunction with FIGS. 2-5, oneconclusion is that the BLEP prediction does not work as intended.Although the CQI reports provide unbiased estimates of the channelquality at the transmission instant, the resulting average BLEP willdiffer from the BLEP predicted from the CQI report. Thus, due to theshortcomings of the BLEP prediction, the performance of the linkadaptation will deteriorate, causing throughput degradation.

To maintain the resulting average BLEP at the desired level. i.e. closeto the BLER target, an adjusted channel quality value, such as a CQIvalue, is generated by the transmitter. The adjusted channel qualityvalue is then used as an input for selecting the Modulation and CodingScheme to be used for transmission to the intended receiver.

In accordance with one embodiment of the present invention an offsetbased on the distribution of the channel quality values is applied tothe channel quality value used. For example if CQI reports are used anadjusted CQI value is formed that is based on the distribution of theCQI reports. The offset can in accordance with one embodiment beconstant but may also be set as a dynamic offset. In a typicallyscenario the introduction of an offset reduces the BLEP for alltransmissions.

In accordance with another embodiment the currently observed averageBLEP can be used as an input parameter when estimating the distributionof channel quality values.

In FIG. 6 a flowchart illustrating steps performed in a transmitter whenselecting Modulation and Coding Scheme is shown. First, in a step 601,the transmitter determines an estimated channel quality value, such as aCQI value. The estimated channel quality value can either be based onactual reports, such as CQI reports, received from a receiver or basedon estimated directly at the transmitter, based on the quality of thetransmissions received from the intended receiver or in some othersuitable manner. Next in a step 603 an adjusted channel quality value isdetermined based on the distribution of the estimated channel qualityvalue. The adjusted channel quality value can be calculated using any ofthe methods described hereinabove such as by applying a constant offsetor by applying a dynamic offset based on for example CQI statistics.Thereupon, in a step 605, a suitable Modulation and Coding Scheme isselected based on the adjusted channel quality value. Then, in a step607, data is transmitted to the intended receiver using the selectedtransmission scheme. Finally, in a step 609, the procedure is repeatedfor the next generated channel quality value and the procedure returnsto step 601.

The procedure as described in conjunction with FIG. 6 can advantageouslybe software implemented by a computer program product 109 loadable intothe modulation and coding selection unit 107 of the transmitter 101,which unit 107 then can execute the program stored on the computerprogram product 109 thereby providing a more efficient transmitter.

Hence, the MCS is selected using a method where the channel qualityvalue is mapped onto the BLEP and where the channel quality value istreated as being associated with an error. This is illustrated in FIG.7.

Using the modulation and coding scheme selection method as describedhereinabove will provide an improved selection of modulation and codingscheme whereby the resulting Block error rate will better correspond tothe block error probability which the transmitter aims at reaching. Byachieving a better correspondence between actual block error probabilityand actual block error probability the transmitter can be used moreefficiently and fewer resources are needed to transmit data from thetransmitter to an intended receiver.

1. A method of selecting a modulation and coding scheme for data to betransmitted from a transmitter to a receiver, where the modulation andcoding scheme is based on a value representing a quality of a channelused for transmission of data from the transmitter to the receiver, themethod comprising: determining an estimated channel quality value,adjusting the estimated channel quality value based on a distribution ofestimated channel quality values, and selecting the modulation andcoding scheme based on the adjusted channel quality value.
 2. The methodaccording to claim 1, where the estimated channel quality value is basedon channel quality index (CQI) reports received from the receiver. 3.The method according to claim 1, where the estimated channel qualityvalue is based on a quality of transmissions received from the receiver.4. The method according to claim 1, where the adjusted channel qualityvalue is formed by adding an offset value to the estimated channelquality value.
 5. The method according to claim 1, where the adjustedchannel quality value is formed by adding a dynamically updated value tothe estimated channel quality value.
 6. The method according to claim 1,where the distribution of the estimated channel quality values isdetermined based on quantization errors.
 7. The method according toclaim 1, where the distribution of the estimated channel quality valuesis determined based on previously received channel quality values. 8.The method according to claim 1, where the distribution of the estimatedchannel quality values is determined based on previously determinedBlock Error Probabilities (BLEP).
 9. A transmitter for transmitting datato a receiver, the transmitter comprising; means for selecting amodulation and coding scheme for the data to be transmitted based on avalue representing a quality of a channel used for transmission of datafrom the transmitter to the receiver, means for determining an estimatedchannel quality value, and means for adjusting the estimated channelquality value based on a distribution of estimated channel qualityvalues, where the means for selecting the modulation and coding schemeselects the modulation and coding scheme based on the adjusted channelquality value.
 10. The transmitter according to claim 9, where theestimated channel quality value is based on channel quality index (CQI)reports received from the receiver.
 11. The transmitter according toclaim 9, where the estimated channel quality value is based on a qualityof transmissions received from the receiver.
 12. The transmitteraccording to claim 9, where the adjusted channel quality value is formedby adding an offset value to the estimated channel quality value. 13.The transmitter according to claim 9, where the adjusted channel qualityvalue is formed by adding a dynamically updated value to the estimatedchannel quality value.
 14. The transmitter according to claim 9, furthercomprising: means for determining the distribution of the estimatedchannel quality values based on quantization errors.
 15. The transmitteraccording to claim 9, further comprising: means for determining thedistribution of the estimated channel quality values based on previouslyreceived channel quality values.
 16. The transmitter according to claim9, further comprising: means for determining the distribution of theestimated channel quality values based on previously determined BlockError Probabilities (BLEP).
 17. A computer program product comprising acomputer program segment that when executed on a computer, causes thecomputer to perform a method of selecting a modulation and coding schemefor data to be transmitted from a transmitter to a receiver, the methodcomprising: determining an estimated channel quality value; adjustingthe estimated channel quality value based on a distribution of estimatedchannel quality values; and selecting the modulation and coding schemebased on the adjusted channel quality value.
 18. The computer programproduct of claim 17, where the estimated channel quality value is basedon channel quality index (CQI) reports received from the receiver. 19.The computer program product of claim 17, where the estimated channelquality value is based on a quality of transmissions received from thereceiver.
 20. The computer program product of claim 17, where theadjusted channel quality value is formed by adding one of an offsetvalue or a dynamically updated value to the estimated channel qualityvalue.